Electric hoist control



May 23, 1961 He Hx. He Hb ik.

Original Filed Nov. 13, 1956 SOHN A. CORTE\ AND Lesue A. KOEN@ Maf ATTORNEY Patented May 23, 1961 ELECTRIC HoIsT CONTROL Original application Nov. 13, 1956, Ser. No. 621,712. Di-

vided and this application, June 22, 1959, Ser. No.

6 Claims. (Cl. S18-209) This invention relates to electric hoist of the class in which a hoist cable winding drum for hoisting and lowering loads is connected to an alternating current motor; and relates particularly to control systems for controlling the speed of hoisting and lowering various loads, by controlling the speed of the motor.

This application is a division of our co-pending application, Serial No. 621,712, filed November 13, 1956.

In a known general class of hoist controls, a polyphase alternating current motor is utilized of the type having a wound rotor or secondary, and external resistance connected in the rotor circuit. A manually operable drum controller is provided having a number of hoisting points and load lowering points at which sections of the resistance are cut in to and out of the rotor circuit. This is done to control the speed of hoisting loads, which may vary from, a heavy load to an empty cable hook, and the speed of lowering loads, which may vary from an empty hook to a load heavy enough to overhaul and drive the motors rotor.

Such controls are not satisfactory. Among the reasons are that the selection of points on the drum controller is optional with the operator. Due to the wide variations of loads, there is no correspondence between the hoisting and lowering points at which the operator sets the drum controller and the speed at which the load hoists or lowers.

Obviously, a heavy load will tend to hoist slowly and lower rapidly, and a light load or empty cable hook will tend to hoist rapidly and lower slowly. It is desirable to hoist and lower all loads at the best speeds dictated by the circumstances and by the height of the load at the time, and to do so in the minimum of time, and without danger. For example, in the case of lowering a heavy load, it may lower so fast as to get out of control and cause damage or injury.

The usual prior controls called for operation of the drum controller by the operator with great care and skill.

`In the development of the art, a particular type of hoist controls has been proposed in which an artificial load, in the form of an eddy current brake, is at all times connected to the motors rotor. The eddy current brake exerts a braking action thereon proportional to the degree of energization of its winding. The winding energization is changed concurrently with changes of the resistance of the motors secondary which are effected by the operators drum controller.

The speed of the motor is controlled partly by its I secondary resistance and partly by the braking action of the eddy current brake; and some of the defects of prior controls are thereby eliminated.

The present invention is an improvement over prior controls of this particular type utilizing an eddy current brake.

The invention is based upon the concept, which is bclieved to be new, that when a hoist control is operated by a drum controller having a number of hoistingA and lowering speed points, the motor should have definite predetermined speeds corresponding to the respective speed points. Each speed should be of a predetermined value and regardless of the amount of the load on hoist cable, the motor should come to these speeds automatically upon setting the drum controller at the set points.

In the embodiment of the invention hereinafter described, it comprises in general the following features and mode of operation; the actual invention being that set forth inthe appended claims.

An eddy current brake has its rotor permanently connected to the rotorof the alternating current motor, and has an energizing field winding, variably energized, to cause it to exert variable braking action on the motors rotor.

A magnetic amplifier circuit of the two stage type is utilized, both magnetic amplifiers having main windings supplied from an alternating current means, and each having control windings to control their output.

The output of the first magnetic amplifier is connected to energize a control winding of the second magnetic amplifier.

One control winding of the first magnetic amplifier is energized at constant value and may be referred to as a reference winding.

The reference winding is polarized to increase the output of the magnetic amplifier. A second control winding is energized by current that may be referred to as signal current and the winding may be referred to as a signal winding. Its polarity is opposite to that of the reference winding and in general is predominated over by reference winding.

The output of the first magnetic ampliiier, and its energization of the control winding on the second magnetic amplifier, is thus caused to vary inversely with the signal current.

The signal current is provided as will be described later.

The output of the second magnetic amplifier is connected to energize the winding of the eddy current brake.

In addition to the control winding on the second magnetic amplifier that is energized from the output of the iirst magnetic amplifier, it has a second winding that is energized at constant value and may be referred to as a bias winding. The polarity of the two windings, control and bias windings, are in opposition with the control winding always predominating. The reference winding is poled to increase the output of the magnetic amplifier.

Since the output of the first magnetic amplifier varies inversely with the signal current, and the output of the first magnetic amplifier is utilized to control and directly increase the output of the second magnetic amplifier, it is evident that the energizationof the eddy current brake winding is caused to vary inversely with the signal current.

The motor rotor has resistance in sections in its external circuit or rotor circuit. These sections may be cut in or out in steps by moving the drum controller to various speed points thereon, causing the current in the rotor circuit to be thereby increased or decreased.

W-hen the current in the rotor circuit is decreased or increased, by the movement of the drum controller, the rotor tends to go faster or slower, respectively, as is well known. That is, the rotor speed varies inversely with the current.

The said signal winding of the magnetic device is connected in a control system such that its energization varies direct-ly with the current in the rotor circuit and inversely with the rotor speed.

In consequence, the output of the magnetic amplifier and its energization of the eddy current brake winding, varies inversely with the current in the rotor circuit and directly with thespeed of the rotor.

Accordingly, if as an example, the drum controller is lmoved to a higher speed point, theV rotor speed'will begin to increase toward a higher value. But at the same time, the signal current will `decrease 7and the output of the magnetic amplifier will increase to increase he energization of the eddy current brake. The brake is thus caused to increasingly oppose the rotation of the rotor so that it will come up to only a certain speed and cannot increase beyond that speed. This being the speed predetermined for the said higher speed .point of the drum controller.

ln this manner at each speed point on the drum Vcontroller, whether for hoisting or lowering, the motor rotor will come to a definite predetermined respective speed 'where it will be maintained at a substantially constant value, regardless of the amount of load on the hoist cable.

The embodiment of the invention as described hereinafter also comprises a mechanically set friction brake for the motor, releasable by an :electric winding. The winding is connected in series with the winding of the eddy current brake to introduce an element of safety. That is, the friction brake will set and stop the motor if the magnetic amplifier circuit energizing the eddy current brake winding, or if the winding itself of the eddy current brake, should fail.

The invention also comprises a lcompensating control 'winding-on the firstmagnetic amplifier that is energized commensurably with the current energizing the eddy current brake. Then in the event of a tendency of energization of the eddy current rbrake to decrease due to any extraneous cause, for example, due to the reduction of the output of the device because of a fall in line potential, or due to an increase of resistance-in the electric lines carrying the output to the brakewinding, the control winding will perform a compensating action and elevate the output of the device and prevent the energization of the brake winding from decreasing; and similarly conversely if it should tend to increase.

The invention also comprises means by which, with the eddy current brake winding and the friction nbrake winding in series, it is insuredthat the friction brake winding will be energized suliiciently to keep the friction'brake released. Vatvery low values of energization of the eddy current brake winding.

The objects of the invention are to provide a jhoist control having, among others, VVthe features land modes of operation above 'described generally, singularly or in combination.

The single figure of the drawing is va schematic -representation of the hoist control of the invention.

Referring to the drawing. there is`shown atM an'alternating current motor having a 3 phase stator '1 and a wound rotor'Z with a 3 phase externalcircuit. having in respective phases, resistancesections R1 to "R5; R6 to Riti; and R11 to R15.

Stator 1 is connected to alternating current supply lines 3, 4, and v5,in which respectively are normally open contacts H1, H2, and H3 of a magnetic contactor to be referred to. When the contactor is operated. the contacts are closed to energize stator 1 to drive rotor 2 in the hoisting direction. Likewise, stator 1 may be connected through the normally open contacts L1, L2, and L3 of another `magnetic contacter, `which when closed, energizes stator `1 v'to drive :rotor 2 in kthe lowering direction. Rotor 2 is connected to a hoist drum 6, and -a cable 7 is wound thereon having'a load electric"electromagnetic winding 10 vfor releasing friction brake'B when energized.

'.mally open contact 2BT1.

Rotor 2 is also connected to a rotor 11 of an eddy current brake EB having an energizing field winding 12; which exerts braking action on rotor 2 commensurably with the degree of energization.

These connections to rotor 2 are diagrammatically represented by the dotted line 13.

At 14 generally is va drum type controller which, in the diagrammatic illustration thereof, comprises movable contacts .15 to 23, all connected together as shown;

'and connected by a wire 24 to current supply line 3.

These contacts, as will be understood by those skilled inthe art, are all movable in unison from the illustrated oli position towards the right to five successive lowering points La to Le, or towards the left to five successive hoisting points Ha to He, indicated by the vertical lines above the legends lowering and hoisting On the several lowering points, movable contacts 15 vand I7 to 23 are engageable with stationary bar contacts 25 to 32 and on the several Ihoisting points, movable contacts 15, 16 and 1S to 23 are engageable with bar con- `tacts 33 to 40.

Drum controller 14 on various lowering and hoisting points, operates or restores electromagnetic contactors as follows.

A contactor ASB having a winding 41 and normally open contacts SBI `to SB4 and normally closed-contacts Aicontactor H having a winding 42 and normally open contacts H1 to H4 and a normally closed contact H5.

A contactor L having a. Winding 43 and normally open contacts L1 to L4 and a normally closed contact L5.

A contactor LB having a winding 44 and normally open contacts LBI to LB4 and a normally closed conwindings 49 to '51 andhave, respectively, normally open contacts '1T1, 2T1, 'and ST1.

A timing coniactor IBT has a winding 53 and a normally open contact 1BT1 and normally closed contact 1BT2. These timing contactors are of a known type that have means delaying their lcontact operation until a time interval has elapsed after'energization of their windings, said means being indicated diagrammatieally as dash pots under the contacts.

A timing contactor ZBT has a Winding52 and a nor- This timing contacter is of thetype that vcloses its contact immediately uponrenergi- `zation of its winding, but delays opening the contact when its winding is de-energized.

The contactors are all illustrated in normally deenergized or restored condition. The said rcontacts of these contactorsare shown without connections thereto, but are reproduced elsewhere in the drawing, with their connections, to thereby Iavoid complexity in the'drawing.

The said Vwindings ofthe contactors are in'an acrossthe-line type of diagram, comprising horizontal cross lines 54 to 68, with theirvright ends connected to acom- \mon wire 69 .going by wire 70 to supply line 4. The

cross lines 58 and 63 to 67 are connected at their left ends to a common Wire 71 going -by wire 24 to supply line 3, and the remaining cross lines, at their left ends are connected to wire 24 and supply line 3 through the contacts of drum controller 14.

A two stage magnetic amplifier circuit consisting of two magnetic amplifiers is shown at 72A and 72B.

The first magnetic lamplifier 72A has input by wires 124 and 125 from a transformer 126 to main windings 146 and 147 and operates to deliver unidirectional output from output points 127 and 128 in response toener- -gization of acompensating winding129, a signal windf ing 130 Vandr a reference lwindingv 131. The second mag- Aneticamplifier 72B-has input-'from a transformer 132"to main winding 76 and 77 and operates to deliver unidirectional output at output points 133 and 134 in response to energization of a bias winding 135 and a control winding 136. Both magnetic amplifiers operating generally in the same manner as will now be described.

At the first magnetic amplifier 72A, signal winding 130 is continuously energized from a full wave bridge rectifier 93 through wires 137 and 13S and adjustable resistor 101. Rectifier 93 receives input from wires 98 and 99 that is derived from the current in rotor resistor R11. Reference winding 131 is energized at constant value from a full wave bridge rectifier 91A through contacts LB2 and adjustable resistor 104 and wire 113A and 114. Rectifier 91A receives input from transformer 126 by wires 124 and 125. Compensating winding 129 is energized by the drop across adjustable resistor 145 in the output circuit of magnetic amplifier 72B and is connected thereto by adjustable resistor 102 and wires 143 and 144. Reference winding 131 is poled in the same ux direction as main windings 146 and 147, and signal winding 130 and compensating winding 129 are poled oppositely thereto with reference winding 131 always predominating.

The output of magnetic amplifier 72A goes from output points 127 and 128 by wires 139 and 140 to energize control winding 136 of magnetic amplifier 72B. Bias winding 135 of magnetic amplifier 72B is continuously energized at a constant value by current in wires 113 and 115 from a full wave bridge rectifier 91, through contacts SB6 or 1BT1 and an adjustable resistor 105. Rectifier 91 receives input from transformer 126. Control winding 136 is polled in the same direction as main windings 76 and 77, and bias winding 135 is poled in opposition thereto, control winding 13-5 always predominating.

The output of magnetic amplifier 72B goes from output points 133 and 134 by wires 107 and 108 to energize windings and 12 of friction brake B and eddy current brake EB through contacts SBI and LB1 or through contacts LB5 and SB5 to resistor 100.

In the illustrated off position of drum controller 14, magnetic amplifier 72A is energized from transformer 126. The current path in `one half cycle is through wire 25, rectifier 88A, main winding 146, output point 127, wire 140, control Winding 136, wire 139, output point 128, rectifier 90A, and wire 124 to the other side of transformer 126. In the other half cycle, the current path is through wire 124, rectifier 89A, output point 127, wire 140, control winding 136, wire 139, output point 128, main winding 147, rectifier 87A and by wire 125 to the first side of transformer 126.

Magnetic amplifier 72B is energized by a transformer 132 and the current path in one half cycle is through wire 106, rectifier 88, main winding 76, output point 133, wire 107, closed contact SB5, resistor 100, closed contact LB5, wire 108, resistor 145, output point 134, rectifier 90, and wire 109 to transformer 132. In the other half cycle, the current path is through Wire 109, rectifier 89, output point 133, and again by the wires 107 and 108 and the circuit connected thereto as described above to output point 134, main winding 77, rectifier 87, wire 106 tothe other side of transformer 132.

On the first point Ha of hoisting, contactor H is operated by current through controller contact 16, bar contact 34, connecting wire 110, cross line 55, contact L5, and winding 42; and closes line contacts H1, H2 and H3, to give current to the motor M. This only occurs if contactor L is restored to close said contact L5.

Contactor SB then operates by current through cont-roller contact 15, bar contact 33, connecting Wire 111, bar contact 25, cross line 54, closed contact H4, and winding 41.

Contact SB3 in cross line 66 closes, energizing Winding 52 of timing contactor ZBT and it operates without time delay, closing contact 2BT1, which is reproduced in cross line 57. Current-then goes through controller Contact 1S, bar contact 35, connecting wire 116, bar con-1 tact 27, cross line 57, closed contacts 1BT2 and 2BT1 and energizes winding 44 of contactor LB and it operates. Contacts SBI and SBS are respectively closed and opened by contactor SB; and contacts LB1 and LBS are respectively closed and opened by contactor LB. This causes the output of magnetic amplifier 72B to be connected directly to windings 10 and 12 of friction brake B and eddy current brake EB in series, so that friction brake B is released and the motor starts.

When contactor LB closed as described above, it closed contact LB2 in the energizing circuit for reference winding 131. Current in one half cycle will then fiow through it from transformer 126, wire 124, rectifier 91A, closed contact LB2, wire 113A, to reference winding 131, and then by wire 114, adjustable resistor 104, rectifier 91, and wire to transformer 126. In the other half cycle, current Hows through wire 125, rectifier 91A, closed contact LB2, wire 113A, reference winding 131, wire 114, adjustable resistor 104, rectifier 91A and wire 124 to transformer 126.

This current will energize reference winding 131 so that magnetic amplifier 72A is caused in the well known manner to produce full output. This it delivers to control winding 136 on magnetic amplifier 72B as previously described.

Control winding 136 being fully energized causes magnetic amplifier 72B to produce its maximum output to insure that winding 10 of friction brake B is strongly energized to positively and quickly release friction brake B.

This is positively insured because during the interval between the operation of contactor SB which opened contact SB6, and the closing of contact 1BT1 of timing contactor 1BT, bias winding 135 of magnetic amplifier 72B is de-energized.

At the end of the time delay of contactor IBT, contact 1BT1 closes the circuit 113-115 of bias winding 135 to energize it in opposition to control winding 136.

This reduces the output of magnetic amplifier 72B to a value for normal operation to be presently described.

At this point, it is interjected that upon going back to the ofi position of drum controller 14 at any time during operation, contactor SB will restore immediately, but restoring of contactor LB will be delayed. This is because it is maintained energized through cross line 58, contact SB7, contact 1BT2 and contact 2BT1. Contact 1BT2 is immediately closed by the restoring of contactor 1BT upon opening of contact SB4, while the opening of contact 2BT1 is delayed by the time delay before contactor 2BT restores after opening of contact SB3.

After the time delay interval of contactor 2BT, contactor LB is restored by the opening or contact 2BT1. During this interval, bias winding 135 of magnetic amplifier 72B is energized through contact SB6 and reference winding 131 of magnetic amplifier 72A is energized through contact LB2. The combined effect, as previously described, is to cause magnetic amplifier 72B to have normal output.

When contactor SB restores, it cuts off energization of winding 10 of friction brake B at contact SB1, and friction brake B immediately sets to stop motor M. However, energization of Winding 12 of eddy current brake EB is maintained through contact SB5, the upper part of resistor 100, and contact LB1 so that during the delay of restoring of contactor LB and opening of contact LB1, the output of magnetic amplifier 72B causes eddy current brake EB to exert braking action, assisting friction brake B in stopping the motor.

Further, upon going to the first point hoisting, contactor 1A operates by current through contact 19, bar contact 36, connecting wire 112, bar contact 28, contact SB2 and winding 45, cutting out rotor resistance sections R5, R10 and lR15 without interposed delay' after operation'of contactor SB.

accesos The motor being energized and running as referred to, current flows in the external rotor resistance and a drop of potential appears across the section R11 which gives input to rectifier 93, by wires 98 and 99, and its output energizes the signal winding 130 through wires 37 and 13S and adjustable resistor 101. This energizing of signal winding i3@ causes the output of magnetic amplifier 72A to decrease, decreasing the energization of control winding 136. Likewise, this decreases the output of magnetic amplifier 72B and its energization of winding l2 to the desired value.

The effects of the above described conditions for the first point of hoisting will be the same whether the motor rotor was running or whether it starts from rest, and the latter condition is chosen for description as being more lreadily understood and is as follows.

Motor stator ll is given line current, friction brake B is released, and eddy current brake EB is energized with normal amplifier output as described; and motor M starts to run.

Due to the high resistance of the roto-r external circuit at tne time, rotor 2 tends to accelerate up to a certain Speed and with small decreasing current in the rotor circuit.

lt is highly desirable in hoists of this class to have a very low speed on the first hoisting point to facilitate manipulation of the empty cable hook into hoisting engagement with the load; and this very low speed is preselected.

While rotor 2 is accelerating, eddy current brake EB will exert braking action on rotor 2 and the braking action will increase directly as the rotor speed increases; whereas the speed of the rotor, while tending to go beyond the preselected speed, will come up only to the preselected speed as follows.

Reference Winding 131 energized at a constant value.

on magnetic amplifier 72A is It produces flux.- in the core of magnetic amplifier '72A in the direction to give high amplifier output to control winding 136. Reference winding 151 is opposed by signal winding 130, but predominates over it. Signal lwinding 150 is energized proportional to the motor current and theresultant flux in magnetic amplifier 7 2A is therefore caused by the differential effect of signal winding 130 and predominating reference winding E51.

The energization of reference winding 131 is adjustable at resistor 101i. By this adjustment, the tiux in the core is preset to a point `on the saturation curve at which small changes in liux, as would be caused by the small changes of the signal current in the signal winding 130, will canse great changes of amplifier output going to control winding 136.

it follows that as the motor accelerates, the rotor current and the consequent signal current fall by small ecreinents, causing the amplifier out, due to amplification, to raise by' very great increments and increase the braking action of eddy current brake EB accordingly until the motor speed cannot increase further. That is, it comes to a definite predetermined speed which as in the premises is an important part of the invention.

This predetermined speed on the first hoist point will always substantially be the same regardless of the amount of load on the cable. With a heavy load, the rotor will tend to rotate more slowly and with greater rotor current, thereby producing correspondingly greater signal current which will produce less eddy current braking action; and vice versa for small loads.

The eddy current braking action is at all loads directly proportional to the rotor speed. Any tendency of the speed to decrease or increase is counteracted by a preponderating decrease or increase of eddy current brake action, and so that the speed is held substantially constant.

A tendency for rotor 2 to increase in speed, reduces the signal current in signal lwinding 130 of magnetic amplifier 72A to increase its output going to control'winding 136 of magnetic amplier 72B. Increase energization of control winding 136 increases the output of magnetic amplifier 72B going to winding 12 of eddy current brake EB. This action continues until the retarding effect of eddy current brake EB is effective to prevent further increase of rotor speed. Thus, the motor comes up to the predetermined speed.

The output of magnetic amplier 72B is controlled by adjusting the energization of bias winding 135 so that when control winding 136 has no energization, its output will be sutlicient to keep friction brake B released. Thus, there is automatically a bottom limit to the output of second magnetic amplifier 72B, determining a minimum braking action by eddy current brake EB, and a maximum predetermined speed, and assurance that friction brake B will be maintained released.

Furthermore, the two magnetic amplifiers 72A and 72B provide in effect a double amplification of the current signal, to which eddy current brake EB responds to maintain constant speed. This renders the response more sensitive to changes of motor speed, and maintaining the constant speed more nearly absolutely constant.

The speed may deviate from absolutely constant speed since some changes of motor speed and consequent change of motor current is required to effect the said regulation; but because of the sensitivity of response of the magnetic amplifiers to changes of signal current and the magnification of changes of output by amplification, the deviation from absolute constancy of speed is maintained within a very small range.

The output of magnetic amplifier 72B to circuit 107- 108 is not affected by the cutting in or out of windings 10 and 12 of friction brake B and eddy current brake EB. The total resistance of resistor is made equal to the combined resistance of the two windings 10 and 12 in series and that of the upper and lower parts of resistor 100 are made equal respectively to that of windings 10 and 12. The contacts associated therewith, as described, are arranged so that when either winding is cut out of the circuit, a corresponding part of resistor 100 is cut into the circuit.

It is desirable to keep the current energizing the eddy current brake winding 12 from changing due to extraneous infiuences; for example: a drop in line potential; or an increase in the resistance of lines 107 and 108 between the amplifier and the brake windings, which lines may in some installations be of considerable length.

To this end, the resistor 145 is placed in the line 108, and the brake energizing current in it produces a drop of potential therein which is utilized in wires 143 and 144 to energize compensating winding 129.

Compensating winding 129 opposes reference winding 131 by an amount adjustable by resistor 102 and at normal values of current in wire 108, its opposition is neutralized by the adjustment of the energization of windings 130 and "131 so that under normal operation it .has no effect.

If, however, the current in the circuit 107-108 should tend to decrease due to some extraneous cause, said opposition by compensating winding 129 would be decreased, causing reference winding 131 to be more effective and prevent the decrease of current.

On going to the second point of hoisting, Hb, the operating conditions described for the first hoisting point remain the same; but, additionally, contactor 2A operates to cut out rotor resistance sections R4, R9 and R14 as follows.

The operation of contactor 1A, as described, closed contact 1A3 in cross line 63, and winding `49 of contacter 1T is thereby energized and ran a timed interval and then operated, closing contact 1T1 in cross line 60.

Current then goes through controller contact 20, bar contact 37, connecting wire 120, bar contact 29, to cross line 60, contact y1T1, winding 46 of contacter 2A and operates it.

This cutting out of rotor resistance sections R4, R9, and R14 causes rotor 2 to speed up toward a higher speed. But, as explained above, the eddy current brake action increases preponderantly with increase of motor speed so that, again, a balance is reached when further increase of rotor speed is prevented by the increased eddy current braking action. Thus, rotor 2 comes again to a definite predetermined speed on the second hoist point, but this time the predetermined speed is higher.

The aforesaid action, producing definite speeds of rotor 2 on drum controller 14 hoist points, is provided on only the first and second hoist points, Ha and Hb. It having been found that that is all that is necessary to meet practical hoisting requirements.

So that in going to the third hoist point, Hc, controller contact 18 leaves bar contact 35, through which operation of contactor LB and energization of eddy current brake winding i12 is effected as described.

Accordingly, contactor LB restores and at brake windings and 12, contact LB1 opens and contact LBS closes, opening the circuit to eddy current brake winding 12 at contact LB1.

The output of magnetic amplifier 72B in lines `107 and 108 then goes through winding 10 of friction brake B alone, by way of contacts SBI and LBS of the lower part of resistor `100, keeping winding 10 energized and friction brake B released.

Restoring of contactor LB opens contact LB2, opening the circuit 113-114 and cutting off energization of reference winding 131.

Restoring of contactor LB also opens contact LB4 in cross line 67 and, in absence of other provisions, would deenergize winding 53, causing contactor 1BT to restore and open the bias winding circuit 113--115 at contact 1BT1. But on this third hoist point, current goes through controller contact 23, bar contact 40, connecting wire 121, bar contact 32, cross line 68, closed contact SB4, and maintains contactor tlBT operated to maintain bias Winding 135 energized.

The de-energization of eddy current brake EB allows the motor to speed up to a higher speed; however, in this instance, the speed is not a predetermined one, but one dependent upon the load on hook 8.

On going to the fourth hoist point, Hd, contactor 3A is operated with delay as follows.

When contactor 2A operated, it closed contact 2A3 in cross line 64 by which winding 50 of contactor 2T is energized. This operated contactor 2T so contact 2T1 closes after a time delay. This contact is in cross line 61. On -fourth point hoisting, current goes through controller contact 21, bar contact 38, connecting wire 122, bar contact 30, closed contact 2T1 and winding 47 of contactor 3A, operating it to cut out rotor resistance sections R3, R8, and R13.

On the fifth hoist point, He, contactor `4A is operated with time delay as follows.

Operation of contactor 3A closes contact 3A3 in cross line 65 and thus causing contactor 3T to operate. After time delay, contact 3T1 closes, which is in cross line 62. On the fifth point hoisting, controller contact 22 is on bar contact 39, and current goes from there through connecting wire 123, bar contact 31, to cross line 62, through now closed contact ST1, to operate contactor 4A, and cut out rotor resistance sections R2, R7, and R12.

Contactors 2A, 3A and 4A therefore are constrained to operate successively on successive hoisting points, each with its own time delay interval.

On the third, fourth, and fth points hoisting, Hc, Hd, and He, contactor `1BT is maintained operated through contact bar 40 as referred to.

On going to the first point of lowering, La, contactor L is operated by current in its winding 43 coming through controller contact 17, bar contact 26, cross line 56 and contact H5. When contactor L operates, it

l0 closes contacts L1, L2, and L3, to supply power to motor M in the reverse or lowering direction.

Thereupon, controller contacts 15 to 18 being on contact bars 25 to 27, the operation of contactors SB and LB occur as described for hoisting.

The conditions and operation is then the same as on hoisting and the motor starts and comes up to the predetermined speed for first lowering point La.

It will be noted that on lowering, contactor 1A is not operated on the `first point as it was for hoisting, therefore, the predetermined speed on first point lowering will be a lower predetermined speed.

On the second point lowering LB, contact 19 engages bar contact 28 and contactor 1A operates by current in cross line 59 to decrease the `resistance in the rotor circuit. This causes motor M to speed up and, likewise, the braking effect of eddy current brake EB increases. Thus, motor M comes up to another and higher predetermined speed in the same manner as described for hoisting.

On the third and fourth lowering points, Lc and Ld, contacts 20 and 21 engage contact bars 29 and 30 successively and contactors 2A and 3A operate, subject to the time delay respectively of timing contactors 1T and 2T as described, and motor M comes up to successively higher predetermined speeds on these points respectively.

On the fifth point of lowering, Le, contact 18 leaves contact bar 27, and contactor LB restores. Energization of eddy current brake EB is cut oif at contacts LB1, but leaves Ifriction brake B energized in the same manner as described for the third point of hoisting.

On the fifth point lowering, also, contact 23 engages contact bar 32 and maintains contactor lBT operated for the same purpose as in hoisting upon engagement of contact `bar 40 by contact 23.

Predetermined speeds are thus provided for on the first four lowering points only.

On the fifth lowering point, Le, contact 22 engages bar contact l31 and contactor 4A operates subject to the time delay of contactor 3T as described for hoisting.

The load on lowering may be a heavy load to over haul and drive the motor rotor, but the current in its secondary is still inversely proportional to its speed. Thus, the signal current is inversely proportional to its speed and as aforedescribed, the eddy current brake action is directly proportional to its speed on the first four points of lowering to hold the motor at speeds predetermined for these respective points.

We claim:

l. In a hoist system, a hoist motor having a rotor, an eddy current brake connected with the rotor and having a winding and exerting braking action on the rotor commensurable with the degree of energization of the winding; a normally set friction brake connected with the rotor and having a winding to release it when energized; the two brake windings connected in series, in a brake circuit; a magnetic amplier circut comprising a first and second Imagnetic amplifier, each having main winding supplied with current from a source and operable to deliver an output; said first magnetic amplifier having a reference winding polarized to increase said rst magnetic amplifiers output and having a signal winding polarized to oppose the reference winding; said second magnetic amplifier having a control winding polarized to increase said second magnetic amplifiers output and having a `bias winding polarized to oppose the control Winding, said control winding connected to be energized by the output of said iirst magnetic amplifier; said brake circuit connected to be energized by the output of said second magnetic amplifier; operable contacts and circuits controlled thereby, to supply source current to the motor and to energize the reference winding to cause the magnetic amplifier circuit to deliver high output to the brake circuit to positively and quickly release the friction brake to let the motor start; connections automatically energizing the bias winding, after a timed interval following energization of thereference Winding to reduce the output of the magnetic amplifier circuit to a normal value.

2. A system as described in claim l and in which, are provided additional operable contacts and circuits controlled thereby to interrupt energization of the eddy current Winding while leaving the friction brake winding energized; and to interrupt energization of the reference Winding, and connections then automatically reducing the second magnetic ampliers output to a value sufficient to maintain the friction brake released.

3. In a hoist control system comprising a hoist motor having primary and secondary circuits, said secondary circuit including an adjustable resistor; an eddy current brake connected with the motor and having a winding and exerting braking action on the motor commensurable with the degree of energization of the winding; a normally set friction brake connected with the motor and having a winding to release it when energized; the two brale windings connected in series in a brake circuit; a magnetic amplifier circuit comprising a first and second magnetic amplifier, each having main windings supplied with current from a source and operable to deliver an output;

`vrsaid first magnetic amplifier having a reference winding t polarized to increase said first magnetic amplifiers output and having a signal winding polarized to oppose the reference winding; said second magnetic amplifier having a control winding polarized to increase said second magnetic amplifiers output and having a bias winding polarized to oppose the control winding; said control Winding connected to be energized by the output of said rst magnetic amplifier; operable contacts and circuits controlled thereby, to supply current to the motor and to energize the reference winding to cause the amplifier to deliver high output to the brake circuit to release the friction brake; connections automatically energizing the bias winding after a time interval following energization of the reference winding; and connections energizing the signal winding directly commensurable with the current in the secondary circuit resistor to cause the magnetic amplifier circuit output to vary and to energize the brake circuit directly commensurably with the speed of the motor.

4. A hoist control system as in claim 3, and having the bias winding energized with a constant value of current so that the second magnetic amplifier Will always have at least a minimum output to the brake circuit to prevent energization of the brake circuit to a value llow enough to allow the friction'brake to set.

5. AA hoist control system as in claim 3, and having means to maintain the eddy current brake winding energized for a predetermined time interval, after the operable contacts are restored and remove current from the motor `and allOW the friction brake to set, whereby both blaks exert braking action on the motor during the said interval.

6. In an electric hoist system, a hoist motorconnected to supply means and comprising a rotor having an external circuit `containing resistance in which current decreases and increases vupon increase and decrease of rotor Speed respectively; ,an eddy current brake having a brake rotor connected with the motor rotor and an energizing winding for exerting braking action on the motor rotor in accordance vWith energization of the winding; a normally set friction brake having a winding for releasing it when energized; the leddy current brake Winding and the frictionbrake winding connected in series; control means comprising a first and second magnetic amplifier energwed from said supply means and each having an output; the firstmagnetic `eunplifier having a reference winding energized at constant value and poled to increase the .first yamplifier output and a ,signal control winding opposing .the reference winding and connected to be variably @,lflrgized bya variable drop of potential in the resistance occurring upon changes of current in the lrotor external circuit; the second magnetic amplifier having a control winding poled Vto increase the second amplifiers `output Y arida bias winding opposing the control winding; the control winding energized by the output of the first .magnetic amplifier; the output o f the second magnetic amplifier energizing the series connected brake windings, `and the bias winding energized from the supply Arriearrs at a constant value sufficient to maintain the `outppt of thesecond magnetic amplifier high enough to keep the frict'i'oncbrake released independently of reduced energiz'ationcf the control winding of the second magnetic anirlrr- References Cited in the file of this patent Publication: Saturating Core Devices by L, R. C row, Edward :Brothers Inc., 1949, page 274. 

